Cathode for use in gas-filled electric discharge devices comprising cadmium, zinc, tin and alloys thereof



- March 19, 1968 R. w. YOUNG 3,374,382

CATHODE FOR USE IN GAS-FILLED ELECTRIC DISCHARGE DEV ICES COMPRISING CADIUM, ZINC, TIN AND ALLOYS THEREOF Filed Nov. 18, 1965 I a m f I VENTOR QODNEVY WILLIRM ouNG B7 JPSC SCLSCI u q ATTORNEYS United States Patent 3,374,382 CATHODE FDR USE IN GAS-FILLED ELECTRIC DISCHARGE DEVICES COMPRISING CADMIUM, ZINC, TIN AND ALLOYS THEREOF I Rodney William Young, Harrow, Middlesex, England, assignor to The M-() Valve Company Limited, London, England, a British company Filed Nov. 18, 1965, Ser. No. 508,463 Claims priority, application Great Britain, Nov. 19, 1964, 47,157/64; June 29, 1965, 27,507/65 12 Claims. (Cl. 313173) This invention relates to cathodes for use in gas-filled electric discharge devices.

According to the present invention there is provided a cold cathode, for use in a gas-filled electric discharge device, at least the emissive part of which consists of a solid material at least 30 atomic percent of which consists of one or more metals having a latent heat of vapourization not greater than 50 kilocalories per mole and a vapour pressure at 1000 C. of not less than 0.1 torr.

By latent heat of vapourization of a metal is meant the amount of heat required to convert one gram-molecule of the metal from a liquid to a vapour at a constant temperature and at the vapour pressure of the system.

Said solid material may suitably consist entirely of one or more metals having properties within the limits specified, but in other arrangements in accordance with the invention said metal or metals may be alloyed with other metal or metals not having properties within the limits specified in order to confer a desired property on the solid material, for example, low melting point.

In a cathode in accordance with the invention said metal or metals may suitably be cadmium and/or zinc, but may also include or consist of magnesium and/or bismuth. Caesium, potassium and barium are other metals having properties within the limits specified but are less suitable owing to their relatively unstable nature.

In a gas-filled electric discharge device incorporating a cathode in accordance with the invention, the gas filling may suitably consist of an inert gas or a mixture of inert gases, but may also consist of or include one or more other gases which will not react chemically with the material of the cathode in operation.

In a particular arrangement in accordance with the invention, the cathode consists of a solid mass of material as specified above adhering to a support consisting of insulating material. The support may suitably form part of the envelope of a discharge device in which the cathode is incorporated.

While not limited to such use, one field of application in which cathodes in accordance with the present invention are especially useful is in gas-filled electric discharge devices of the kind which, hitherto, have utilised a cathode in the form of a pool of conducting liquid. In such an application a cathode in accordance with the invention has the advantage that it enables the device in which it is incorporated to be operated in any position and under conditions of appreciable vibration.

Another field of application in which cathodes in accordance with the invention are especially useful is in gas-filled electric discharge devices of the kind incorporating an electrode structure including an anode, a cathode, and a trigger electrode interposed between the cathode and anode, the arrangement being such that a discharge between the anode and cathode may be initiated by the application of a suitable triggering voltage between the trigger electrode and the cathode. The use of a cathode in accordance with the invention in such a device has been found to enable the device to be triggered at lower triggering voltages than with devices of similar construction but incorporating known forms of solid cold cathode.

of one centimetre, the

One gas-filled electric discharge device incorporating a cathode in accordance with the invention and suitable for operation as a surge diverter will now be described, by way of example, with reference to the accompanying drawing which is a sectional view of the device.

The device has a sealed glass envelope 1 within which are disposed a cathode 2, an anode 3 and a trigger electrode 4. The envelope 1 is generally of circular cylindrical form, the cathode 2, which is in the form of a blob of material having a volume of about two cc. and consisting of cadmium, zinc or an alloy of cadmium and zinc, being disposed at one end of the envelope 1 in direct contact with the inside of the envelope 1. At this end, the envelope 1 is narrowed down to form a hollow projecting pip 5, the cathode 2 filling the space within the pip 5 and extending into the space above it; a connector for the cathode 2 is provided in the form of a tungsten rod 6 which is sealed through the end of the pip 5 so that one end of the rod 6 is within the blob of material constituting the cathode 2.

At its other end the envelope 1 is formed with a reentrant portion 7, through which is sealed an assembly including a glass tube 8 and a tungsten rod 9 which are disposed coaxial with each other, the rod 9 having a circular cross-section of diameter four millimeters and the tube 8 having a circular cross-section with internal and external diameters respectively of 6.5 millimeters and one centimeter. The outer end of the tube 8 is sealed onto the rod 9 at a'p-oint near the outer end of the rod 9, while the tube 8 is sealed through the portion 7 of the envelope 1 at a point about midway along its length. The inner end of the tube 8 projects for a distance of one millimeter beyond the inner end of the rod 9, the portion of the rod 9 adjacent this end elfectively consituting the anode 3 of the device. It will be appreciated that the remainder of rod 9, which has a total length of about eight centimeters, constitutes a connector for the anode 3. The trigger electrode 4 includes a nickel tube 10 havmg -a length .of 1.5 centimetres and an internal diameter tube being provided at one end with an outwardly projecting radial flange 11 and being disposed so that it fits around the tube 8 with its flanged end in register with the inner end of the tube 8; the tube 10 may for example be formed by bending round a fiat sheet of nickel and clamping the free edges together. The hanged end of the tube 10 is closed by a closure member in the form of an iron disc 12 having a diameter substantially equal to the outer diameter of the flange 11, the disc 12 being welded at its periphery to the flange 11 and having one main face abutting against the end of the tube 8 so that the trigger electrode 4 is accurately located in a longitudinal direction. The disc 12 has formed at its centre a circular aperture 16 of diameter one millimetre. It will be appreciated from the dimensions given above that the spacing between the disc 12 and the adjacent end of the rod 9 is one millimetre and the spacing between the rod 9 and the tube 10 is three millimetres. It will be further appreciated that the spacing of the tube 8 from the rod 9 over a length of the rod 9 contiguous with the anode 3 reduces the risk of breakdown occurring between the anode 3 and the trigger electrode 4 through or along the surface of the tube 8. The total length of the envelope 1 is such that the spacing between the cathode 2 and the trigger electrode 4, which is not critical, has a value of :a few centimetres, and a connector tor the trigger electrode 4 is provided in the form of a metal rod 14 sealed through the side wall of the envelope 1 at a position which is more distant 'from the cathode 2 than is the trigger electrode 4, the trigger electrode 4 being connected to the rod 15 by a suitable lead 15.

The envelope 1 is filled with argon at a nominal pressure of 0.29 torr, the tolerance allowed on this parameter during manufacture being about The value of the anode breakdown voltage for a device having the construction described above and with an argon filling at a pressure of 0.29 to'rr is about fifteen kilovolts with the trigger electrode 4 at the same potential as the cathode. In this connection it is pointed out that in the device described, by way of example, the deposition of material from the cathode onto the anode-trigger electrode structure does not lead to appreciable reduction in the anode breakdown voltage, as is the case in a similar device employing a mercury pool cathode. The triggering characteristics of the device described, by way of example, may be illustrated by considering a case in which the device is triggered by means of a pulse corresponding to one half cycle of a sinusoidal alternating voltage of firequency 50 kc./-s. and amplitude two kilovolts, the pulse being applied between the trigger electrode 4 and the cathode 2 so as to drive the trigger electrode 4 more positive and the current flowing through the trigger electrode 4 being limited to a maximum value of 10 milliamperes. With such an arrangement it is found that a discharge between the anode 3 and cathode 2 may be reliably initiated in the device described above with values of the applied anode-cathode voltage down to less than 500 volts.

In an otherwise similar device having a molybdenum cathode reliable triggering can be achieved only with triggering voltages of at least four kilovolts.

The improved triggering characteristic is attributable to the low latent heat of vapourization of the cathode material; as a consequence of this, cadmium and/ or zinc vapour is readily produced as soon as a glow discharge in argon is initiated in the valve, thus giving a high vapour pressure in the valve which in turn leads to rapid formation of an arc discharge between the trigger electrode and the cathode.

Finally, it may be noted that the device described above is capable of passing peak anode currents having a value of 2000 amperes.

In a modified form of the device described above by way of example the cathode consists of an alloy containing 33.6 At percent cadmium and 66.4 At percent tin. The electrical properties of the device are substantially the same as those of the device described, by way of example, but the material of the cathode melts at a relatively low temperature compared with cadmium, zinc and alloys of cadmium and zinc, thus facilitating formation of the cathode.

'I claim:

1. A cold cathode, for use in a gas-filled electric discharge device, at least the emissive part of which consists of a solid material at least 30 atomic percent of which consists of metal having a latent heat of vapourization not greater than kilocalories per mole and a vapour pressure at :1000 C. of not less than 0.1 torr.

2. A cathode according to claim 1, the cathode consisting entirely of said solid material.

3. A cathode according to claim 2, the cathode consisting of a mass of said solid material adhering to an insulating support.

4. A cathode according to claim 3 wherein said insulating material is glass.

5. A cathode according to claim 3 incorporated in a gas-filled electric discharge device having an envelope, part of which envelope constitutes said support.

6. A cathode according to claim 1 wherein at least part of said metal is zinc.

7. A cathode according to claim 1 wherein at least part of said metal is cadmium.

8. A cathode according to claim 11 wherein said solid material consists entirely of metal having properties within the limits specified in claim 1.

9. A gas-filled electric dischrage device incorporating a cold cathode according to claim 1.

10. A gas-filled electric discharge device according to claim 9 having a filling of inert gas.

1 1. A gas-filled electric discharge device incorporating an anode, a cathode according to claim 1, and a trigger electrode interposed between the anode and cathode, the trigger electrode serving to initiate a discharge between the anode and cathode on application to it of a suitable potential.

12. -A gas-filled electric discharge device according to claim 11 wherein the device has an envelope of insulating material and said cathode consists of a mass of said solid material adhering to the internal surface of a part of said envelope.

References Cited UNITED STATES PATENTS 3,089,054 5/1963 Walsh ct a1 313-2l8 X 3,183,393 5/1965 Peterson 313-311 X 3,317,776 5/1967 Brittain et al. 313-328 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. A COLD CATHODE, FOR USE IN A GAS-FILLED ELECTRIC DISCHARGE DIVICE, AT LEAST THE EMISSIVE PART OF WHIC CONSISTS OF A SOLID MATERIAL AT LEAST 30 ATOMIC PERCENT OF WHICH CONSISTS OF METAL HAVING A LATENT HEAT OF VAPOURIZATION GREATER THAN 50 KILOCALORIES PER MOLE AND A VAPOUR PRESSURE AT 100*C. OF NOT LESS THAN 0.1 TORR. 